1. Field of the Invention
The present invention generally relates to slack adjusters for vehicle brakes and, more particularly, to an improved automatic slack adjuster for an air brake system of the type commonly used on heavy trucks and construction machines
2. Description of the Prior Art
Heavy vehicles, such as trucks, are typically provided with an air brake system. An air brake system generally includes an air chamber to which air is supplied during braking, an S-cam mounted to one end of a rotatable shaft and coupled to brake shoes for causing expansion thereof against a brake drum upon rotation of the cam shaft in a given direction, a slack adjuster mounted to the other end of the cam shaft, and a push rod extending from the air chamber and pivotally connected to the slack adjuster for causing pivoting thereof and rotation of the cam shaft upon extension of the push rod by supply of compressed air to the air chamber.
Thus, when the pedal of the brake system is applied for braking, compressed air is supplied to the chamber causing extension of the push rod from the chamber through a working stroke against the bias of a return spring. Extension of the push rod results in pivoting of the slack adjuster to an applied position and rotating of the S-cam which, in turn, expands the brake shoes into contact with the brake drum. Then, when the brake pedal is released, compressed air is vented from the chamber permitting retraction of the push rod into the air chamber through a return stroke due to the force of the return spring. Retracting of the push rod results in reverse pivoting of the slack adjuster back to its released position and reverse rotating of the S-cam which, in turn, retracts the brake shoes clear of the brake drum. In such manner, one brake apply and release cycle is completed.
Ideally, a minimum clearance is provided between the brake shoes and the brake drum when the brake is released. But as brake linings wear over time or the brake drum expands due to heat, the clearance between the brake shoes and the brake drum increases. As a result, the air chamber push rod has to be extended or pushed further to pivot the slack adjuster and rotate the cam shaft to compensate for the increased clearance. However, the air chamber push rod travel is inherently limited. Therefore, adjustment of the slack adjuster is required to compensate for the increased clearance.
The required adjustment of the slack adjuster is commonly done by manually rotating a worm and worm gear in the slack adjuster. Rotation of the worm gear, in turn, rotates the cam shaft and advances the S-cam contact point to restore the proper running clearance between the brake shoes and the brake drum. However, manual adjustment is generally undesirable since relatively frequent adjustment is required as brake linings become worn.
Automatic slack adjusters for vehicle brakes are known in the prior art. Various techniques are provided in the prior art slack adjusters for rotating the worm gear in response to increase in the push rod working stroke to automatically change the angular position between the slack adjuster and the cam shaft for eliminating slack due to wear of brake linings. (Automatic slack adjusters should not adjust to take-up slack due only to thermal expansion because the brakes could lock-up when cooled-down.) However, problems have occurred with prior art automatic slack adjusters due to poor reliability, excessive weight, complicated and expensive construction and in providing a design which accurately distinguishes needed adjustment for brake lining wear from unwanted adjustment for thermal expansion to establish a desired clearance between the brake shoes and the brake drum when the brake is released. Thus, improvement is still needed in the design of an automatic slack adjuster.